simple string compressor

stringcompress.cpp

//examples: 
//  compress("AABBBCCCCCAADDDD") == "2A3B5C2A4D"
//  compress("PPPQRRRSTTQQS") == "3PQ3RS2T2QS"
//  compress("uvw") == "uvw"

#include<sstream>
#include<string>

void _format(std::ostringstream& out, size_t counter, char current)
{
    if(counter == 1)
        out << current;
    else
        out << counter << current;
}

std::string compress(std::string const& input)
{
    if(input.size() == 0)
        return std::string();
    std::ostringstream out; 
   
    size_t counter = 0;
    char current = input[0];
    for(std::string::const_iterator it_char = input.begin(); it_char != input.end(); ++ it_char)
    {
        if(*it_char == current)
            ++ counter;
        else
        {
            _format(out, counter, current);
            current = *it_char;
            counter = 1;
        }
    }
    _format(out, counter, current);
    return out.str();
}